A voltage of a frequency three times the frequency of the network, referred to below as the third harmonic, is generated in different ways in distribution and transmission networks, commonly referred to below as power networks. This voltage generates a third-harmonic current which often causes great problems, on the one hand for apparatus and attachments connected to the power network, and on the other hand for the third-harmonic generating devices themselves. The third-harmonic problems are often directly associated with the way the connected devices are grounded. Because there are several different principles and regulations for grounding in power networks it follows that the methods for reducing influence from third harmonics can be extremely different. The present invention deals with how to reduce the problems with third-harmonic currents that may arise during generator and motor operation of AC machines. The invention consists of a method and a connection arrangement for achieving the above mentioned.
When calculating and designing three-phase AC machines, the aim is normally to achieve as symmetrical and sinusoidal quantities as possible. With respect to terminals the stator windings of the above-mentioned machines may, when being manufactured, be connected in many different ways. For some machines the stator windings are connected in xcex94, for others in Y-connection, where the so-called neutral point sometimes is not drawn out from the machine. For the machines concerned in this invention the three-phase winding is Y-connected and all of the winding ends including the neutral point are drawn out from the machine. In order to obtain an economic yield from the electromagnetic circuit in common types of AC machines, a third-harmonic EMF is generated as a harmonic to the fundamental EMF.
It is well known that the chording of the stator winding may be chosen in order to eliminate one or more of the harmonics. It is also well known regarding synchronous AC machines with salient poles that, in addition, the shape of the EMF of these machines may be influenced and improved by choosing the design of the rotor poles and, especially, the shape of the pole shoes in an appropriate way.
A total elimination of the third harmonic of the voltage by choosing an appropriate size for the winding step however means a considerable reduction, approximately 14%, of the fundamental frequency voltage available for torque generation. This thus means only 86% utilization of the possible rated power. In order to avoid this reduction, the winding step is used mainly for suppression of the fifth harmonic whereby the reduction becomes only a few percent. Adaptation of the shape of the pole shoe is often used for a cost-effective reduction of the seventh harmonic voltage. Elimination or reduction of the harmful effects of the third-harmonic voltage/current must thus be performed by different methods.
When a generator is to be connected directly to an existing power network the grounding of the generator cannot normally be freely chosen due to the fact that this is mainly determined by the grounding method of the existing power network. Concerning grounding, however, there are important criteria which should be fulfilled, namely:
the power network may be directly grounded, resonance-grounded, ungrounded, high-impedance grounded
a third-harmonic current through the generator as well as other equipment connected in the power network must for many reasons be limited
If the neutral point is connected to the ground of the power network by a relatively high-ohmic impedance for both the fundamental and the third component, increased voltages relative to ground will arise on the unfaulted phases in case of a ground fault. This cannot be accepted on certain markets.
By connecting the neutral point directly, or via an appropriate impedance, to the ground of the power network, this will however allow the AC machine concerned to contribute to obtain appropriate magnitudes of the zero-sequence impedances to be able to handle the fault conditions, for example voltage increases on unfaulted phases, which may arise in the electrical system.
Traditionally, certain criteria should be fulfilled concerning the zero-sequence impedance R0+jX0 and positive-sequence reactance X+ of the systems. These criteria are often denoted as the following inequalities and state that
X0/X+ less than 3 and R0/X+ less than 1 
In other respects, these inequalities can be interpreted in such a way that in case of a ground fault in one phase, the voltage increase, in the unfaulted phases relative to ground, can be limited to 80%, which is an economical value from the point of view of insulation coordination, of what would have occurred if X0and/or R0xe2x86x92∞.
The disadvantage with a direct connection of the neutral point to the ground of the power network is that, if the voltages contain a third harmonic, a third-harmonic current will start to flow in the phase conductors which closes its circuit through the ground and/or the neutral conductors. It can be mentioned that there are no regulations prohibiting such an arrangement and that there are such installations in operation.
Concerning low-voltage power networks, there are today in most networks third-harmonic currents which close their circuit through the neutral conductor and cause thermal as well as acoustic problems. The occurrence of and a method of reducing the influence of these third-harmonic currents will be described below.
When it comes to limiting the detrimental effect of the third-harmonic voltage and the third-harmonic current, there are a number of different methods in addition to grounding of the neutral point by means of appropriate impedances.
A relatively common method of grounding is to connect a high-ohmic resistance to a point on the power network which is always connected. This can be done by means of a Z/0-connected transformer connected to the network. To obtain the required selective ground fault protection devices, however, the resistance should be dimensioned such that at least, a ground fault current at full neutral point voltage is obtained.
Another common method for handling the third-harmonic problems for generator plants where the neutral point is available and which, in addition has obvious advantages with regard to limiting fault current in case of a ground fault in the generator appears from the accompanying FIG. 1. A generator 1 has the neutral point 2 via an impedance 3, low ohmic resistor often a neutral-point resistor dimensioned for ground fault current of some harmless 10-20 amperes or so, connected to the ground of the power network 4. A ground fault in the generator thus can cause a ground-fault current via the impedance, and by controlling the ground fault current, measures can be taken to disconnect the generator or a possible defective phase. The phase voltages of the generator are connected to a xcex94/Yxe2x88x920 connected so-called step-up transformer 3 which has to be dimensioned for full power even if it should not be reason to change the voltage level. A ground fault in the A-winding of the transformer is limited, in the same way as for a ground fault in the windings of the generator, to some 10-20 amperes. The third-harmonic voltages with which the generator is afflicted could give a third-harmonic current. However, the third-harmonic voltage is superimposed on the phase voltages of the E-winding but cannot generate any third-harmonic current via the neutral-point resistance to ground. This means that, on the Y-side of the transformer, i.e. on the power network side, no third-harmonic current is sensed. In FIG. 1 the necessary auxiliary power input from the network and the field excitation of the generator are omitted. As is otherwise clear from the figure, current measurement devices 6 and 7 are needed for the ground fault current and for the current delivered from the generator. On the generator side, in addition, both a disconnector and a circuit breaker 8 are needed. For the step-up transformer both current measurement 9 and 10 are needed for incoming and outgoing currents. In addition at the input side of the step-up transformer an overvoltage protection device 11 is needed. The network side is then grounded in a normal way via the neutral point of the Y-side. Also here a current measurement device 12 is needed. On the network side both a disconnector and a circuit breaker 13 are then needed as usual.
The EPRI report EL-3391 xe2x80x9cHigh Voltage Stator Winding Developmentxe2x80x9d describes in Section 4 the interplay with the power network in case of direct connection of superconducting high-voltage generators. 4.1 describes the preferred embodiment, in this publication, of xcex94-connected generators, whereby the third-harmonic problems in the power network disappear through the currents circulating in xcex94. 4.1.1 and 4.1.2 describe grounding methods. The low zero-sequence impedance is ensured via a shunt-connected transformer.
From what is stated above it is obvious that concerning Y-connected direct-connected high-voltage electric machines, the question of third harmonics is a problem which is not satisfactorily solved.
As mentioned above, low-voltage distribution networks contain an obvious third-harmonic current which is difficult to remedy. This is due to the fact that modern fluorescent lamp fittings, thyristor controllers and power supply units chop or load the line voltage non-linearly. These disturbances influence other apparatus, cause power losses and may affect persons hypersensitive to electricity. The disturbances have an obvious third-harmonic nature. In order to reduce the disturbances there is a filter which is described in ERA 8:1994, page 26, and which is installed into the neutral conductor and means that the effect of the harmful third harmonic disturbances can be drastically reduced.
To be able to cope with the problems which arise from third-harmonic voltages in case of direct connection of AC machines to a three-phase power network, i.e. when there is no transformer between the AC machine and the three-phase network, according to the invention a special method is used and a connection arrangement, which comprises connecting a suppression filter tuned to the third harmonic between the neutral point in the Y-connected AC machine and the ground of the power network, possibly via a very low-ohmic resistor. The filter comprises a parallel resonance circuit which consists of a reactor with the fundamental reactance XL connected in parallel with a capacitive reactance with a fundamental value xe2x88x92jXC. The ratio between the reactances to achieve third-harmonic parallel resonance will thus be
3XL=XC/3 
To limit the voltage across the filter to economic values a conventional overvoltage protection device is included, as an integral part of the filter, and is connected across the parallel resonance circuit. An overvoltage protection device connected in this way also limits the voltage at the neutral point to such values that a tapered insulation can be utilized in the stator winding. This is important especially in very high-voltage AC machines.
The dimensioning of the filter and its overvoltage protection device shall be such that the parallel circuit is capable, during normal operation, of linearly absorbing the third-harmonic voltage from the AC machine. In the event of a fault the overvoltage protection device is to limit the fault voltage so that the fault current is allowed to flow through the overvoltage protection device if the fault is considerable.
There are a number of considerable advantages with an arrangement according to the invention. As is clear, among other things, from the accompanying FIG. 2, which shows an embodiment of the invention, the step-up transformer as well as the associated current measurement devices and circuit-breakers and disconnectors are eliminated.
Because the filter will only be loaded by zero-sequence quantities and even if there will occur a large circulating third-harmonic resonant current, the rated power of the filter will be essentially lower than the rated power of the step-up transformer. This means that a solution to the third-harmonic problems according to the invention will be considerably less cost-demanding than a solution according to the prior art.